This invention relates to a method and an apparatus for producing an optical element, such as a lens, made of an optical glass and, in particular, to a method and an apparatus for producing a glass optical element by press-forming or press-molding a heated and softened glass material with high accuracy using a forming mold or die precisely shaped in conformity with a desired shape of the optical element. This invention also relates to a glass optical element produced by the method and the apparatus.
In order to produce an optical element, such as an optical glass lens, for use in an optical apparatus such as a camera and an optical pickup, proposal has recently been made of a number of methods in which a heated and softened glass material is press-formed or press-molded by the use of a forming mold made of metal or ceramics. In this case, use may be made of glass materials (i.e., preforms preliminarily shaped) in various shapes, such as a spherical shape, a rod-like shape, and a flattened spherical shape. In case where the glass materials of these shapes are used to form optical elements, there may arise a problem which will presently be described. Referring to FIG. 1, depending upon the relationship between the shape of a glass material W and the shape of the forming mold, i.e., the shape of an optical element to be formed (for example, if a radius of curvature of the glass material W is greater than a paraxial radius of curvature of a forming surface of the forming mold), a space S may sometimes be formed between a lower die 2 and the glass material W. If press-forming is carried out in a state where a gas trapped in the space S is not discharged, a glass surface of the optical element obtained by press-forming undesirably has a recess, called a gas trap mark, formed at a portion corresponding to the space S where the gas remains trapped. As a result, optical performance and surface quality of the optical element obtained by press-forming are adversely affected.
In order to remove the above-mentioned problem, various conventional techniques have been proposed as follows.
Japanese Unexamined Patent Application Publication (JP-A) No. H6-9228 (Reference 1) discloses a forming method comprising the steps of press-forming an approximate half of a total deformation amount by heating and pressing a material with a pressing pressure reduced or released at least once during heating and thereafter press-forming the rest of the total deformation amount by cooling and pressing the material.
Japanese Unexamined Patent Application Publication (JP-A) No. H8-325023 (Reference 2) discloses a method in which, upon press-forming a flattened glass material, a gas present between a forming surface and the glass material is allowed to escape outward through a groove or a protrusion formed at a topmost portion of an outer periphery of the forming surface.
Japanese Unexamined Patent Application Publication (JP-A) No. H11-236226 (Reference 3) discloses a method in which a forming chamber is evacuated in a pressing step.
Japanese Unexamined Patent Application Publication (JP-A) No. H8-245224 (Reference 4) discloses a method in which, immediately before a heated and softened glass material is press-formed, a space around the glass material is reduced in pressure.
However, the above-mentioned methods are disadvantageous in the following respects.
In the method disclosed in Reference 1, the pressing pressure is reduced or released during heating so that the gas trapped in the space is returned into a normal pressure. In this method, however, the gas tends to remain trapped in the space depending upon the shape or the volume of the space. In order to completely discharge the gas, the pressing pressure must be repeatedly increased and decreased. In addition, mold release at a press-forming temperature may cause glass fusion or defective appearance of an optical element obtained by press-forming.
In the method disclosed in Reference 2, the groove or the protrusion is formed at the topmost portion of the outer periphery of the forming surface to allow the gas present between the forming surface and the glass material to escape outward. In this method, however, the shape of the groove or the protrusion is transferred onto the optical element obtained by press-forming. As a result, the optical element has a deformed part, for example, at an attaching portion to be attached to an optical apparatus. In some cases, a post-processing step of removing the deformed part is required.
In the method disclosed in Reference 3, heating is carried out after evacuating the forming chamber to discharge the gas trapped in the space. In this method, however, it is impossible to utilize heat conduction through an atmospheric gas in the forming chamber as a medium in order to heat the forming die and the preform because the forming chamber is evacuated into vacuum. As a result, heating efficiency is insufficient. In addition, heat-soaking to bring the die and the preform to uniform temperature is difficult and temperature control is unstable.
In the method disclosed in Reference 4, the glass material is heated to a temperature not lower than a softening point and thereafter transferred by a conveying member to a position between upper and lower dies. After the space around the glass material is reduced in pressure, the glass material is press-formed. In this method, the glass material is transferred to the position between the upper and the lower dies after the glass material is heated to the temperature adapted to press-forming. As a result, the glass material has a low viscosity and is inevitably deformed to trap the gas between the glass material and the lower die after the glass material is transferred. Therefore, the gas can not be purged or removed even if pressure reduction is performed thereafter.
Thus, in the above-mentioned conventional methods described in References 1 to 4, it is impossible to completely purge the gas from the space between the forming mold and the preform.
In the meanwhile, in an optical pickup for use with an optical information recording medium to record and/or reproduce information, an objective lens having higher resolution is required with an increase in recording density. Therefore, in the optical pickup, it is required to use a light source having a short wavelength and an objective lens having a large numerical aperture (NA). Also as a coupling lens for optical communication, a high-NA lens is required in order to improve coupling efficiency.
In the high-NA lens, however, a surface tilt angle (an angle formed between a normal to a lens surface and an optical axis) tends to be large at its periphery. For example, the surface tilt angle may exceed 40°, sometimes may be as large as 50° to 63°. In addition, a lens useful as the high-NA lens tends to have a small paraxial radius of curvature. Further, in order to assure a production tolerance or to reduce wavefront aberration, the high-NA lens may be increased in center thickness. As a result, the glass material is increased in volume.
For example, in case where the preform as the glass material has a spherical shape, RM/RL is greater than 1 and has a value within a range of 1.0<RM/RL≦1.6, in particular, 1.1≦RM/RL≦1.6, where RM represents a radius of curvature of the glass material and RL represents a paraxial radius of curvature of the lens (paraxial radius of curvature of the forming surface of the forming mold). Thus, the glass material is increased in volume within a range such that an outer diameter of the glass material does not exceed an outer diameter of the lens. If the lens of such a shape is formed, a space is inevitably formed between the forming surface and the glass material.
However, even in case where RM/RL is large as mentioned above, it is required to form a lens excellent in optical performances and surface quality.